1. Field of the Invention
The present invention relates to a novel reinforced rubber composition having an excellent productivity, processability, and yet an excellent modulus, strength and crack growth resistance of a vulcanized product thereof, and a process for the production thereof.
The excellent characteristics of the reinforced rubber composition of the present invention enable it to be used for products such as tire inner members, such as the belt, carcass, bead, etc., tire outer members, such as the tread, sidewall, etc., rubber parts for automobiles, industrial products such as belts, hoses, etc., and as footwear materials, etc.
The present invention also relates to a bead filter rubber composition having an excellent processability with a small Mooney viscosity (ML), which can produce a vulcanized product having a good flexural crack growth resistance.
The present invention further relates to a tire for an automobile in which a rubber composition having an excellent processability and a small Mooney viscosity (ML.sub.1+4) is used for the cap tread rubber.
The present invention still further relates to a rubber crawler which has an improved cut resistance and moldability and by which the weight of an article is reduced.
2. Description of the Related Art
In the prior art, reinforced rubber compositions to be used for respective members of tires have been produced by formulating short fibers of nylon, polyester, vinylon, etc. with a vulcanizable rubber. However, the thus obtained reinforced rubber compositions have greater fiber diameters and no bond between fibers and the rubber. Accordingly, the vulcanized product does not have a sufficient modulus and strength, particularly the modulus and strength at high elongation, and there is a demand for the development of a reinforced rubber composition having a greater modulus and strength.
Accordingly, an elastomer reinforcing process comprising kneading a polymer capable of fiber formation with a rubber and then extruding the kneaded product, and a reinforced elastomer composition, have been proposed (see Japanese Unexamined Patent Publication (Kokai) No. 53-8682). However, according to this process, thick fibers or films of polymers are formed, and because of an absence of bonding between rubber and the polymer (fibers) at the fiber interface, the tensile strength and modulus at a high elongation and the peel-off strength, i.e., adhesive force with other various substrates are low, and thus a reinforced rubber composition usable for the preparation of tire members cannot be obtained.
Japanese Examined Patent Publication (Kokoku) No. 55-41652 discloses a process for the production of a reinforced rubber composition comprising kneading a vulcanizable rubber with powdery 1,2-polybutadiene, extruding the kneaded product, and then rolling the extrudate with rolls. The reinforced rubber composition according to this process, although the vulcanized product thereof exhibits high modulus, high elongation, high strength, and high peel-off strength, has a low strength of the fibers formed from 1,2-polybutadiene and, therefore, during rubber processing, particularly during kneading with carbon black, fibers are cut and the fiber lengths shortened, and thus a drawback arises in that the stress at low elongation of the vulcanized product is small.
Accordingly, to improve the drawbacks of the reinforced rubber compositions known in the art, the present inventors have proposed a process for the production of a reinforced rubber composition comprising melt extruding a composition containing a block copolymer of liquid diene type rubber and nylon formulated in a vulcanizable rubber to form nylon into fibers, and at the same time, graft the vulcanizable rubber to the liquid diene type rubber (Japanese Unexamined Patent Publication (Kokai) No. 58-19342). However, this production process does not have a good reproducibility of the block polymerization and has a high cost, and thus problems arise in the practical application thereof.
Further, the present inventors, to obviate these drawbacks, proposed a reinforced rubber composition including fine polyamide fibers buried in a vulcanizable rubber and having a vulcanizable rubber graft bonded to the fibers at the fiber interface through a precondensate of phenol formaldehyde (Japanese Unexamined Patent Publication (Kokai) No. 60-186550).
However, although this reinforced rubber composition has an excellent productivity, processability and can give a vulcanized product having an excellent modulus, tensile strength and adhesiveness, etc. at a low elongation and high elongation, a drawback arises in that the crack growth resistance is low, and therefore, the uses thereof have been remarkedly limited.
EPDM rubber, which is an ethylene-propylene-diene copolymer, has an excellent heat resistance and weathering resistance, and has been utilized widely for rubber products for industrial uses, such as a material for heat resistant rubber hoses, heat resistant rubber belts, rubber vibration isolators, sponge rubbers, etc. When an ordinary EPDM rubber is provided for the production material of the above products, various reinforcing agents are formulated, depending on the kind of product. Examples of such reinforcing agents may include inorganic reinforcing agents such as carbon black, silica, magnesium carbonate, magnesium silicate, and organic reinforcing agents such as phenol-formaldehyde resin, polyamide fiber, aramide fiber, polyester fiber, etc.
The EPDM rubber composition formulated with the above reinforcing agent has an increased green strength in the unvulcanized state, and the vulcanized product exhibits a high modulus, but a large amount of the reinforcing agent must be formulated to increase the effect thereof. However, when a large amount of the above reinforcing agent is formulated, the flowability and roll processability are reduced, and the surface smoothness of the extrudate during extrusion molding is lowered, and therefore, productivity is lowered and the practical value of the product is reduced. Also, among the above organic reinforcing agents, when an organic fiber is used as the reinforcing agent, ordinarily an RFL treatment is applied to the above organic fiber to enhance the adhesiveness or close contact of said organic fiber with EPDM rubber, but an adequate RFL treatment method has not been developed as yet.
For the above reasons, the above reinforcing agent cannot be formulated in a large amount in EPDM rubber, and therefore, the green strength of EPDM rubber cannot be sufficiently increased. Accordingly, in the production steps of rubber products for extrusion, a cumbersome step of semi-vulcanization is incorporated in the course thereof, a mandrel to prevent mold deformation is inserted, or special vulcanization equipment is required, and thus the steps become complicated and cause an increase in costs.
Also, due to the absence of an adequate RFL treatment method, the interfacial bond between organic fiber and EPDM rubber is weak, and in the tensile test of the vulcanized product, voids are generated in the vicinity of the interface of the above organic fiber in the elongation process, with the result that the tensile strength is lowered and elongation at break is reduced.
Also, in an EPDM composition formulated with an inorganic reinforcing agent with a low reinforcing effect, such as silica, magnesium carbonate, magnesium silicate, calcium carbonate, clay, alumina, etc., because of a low green strength, special techniques and equipment are required for the production of hollow products or extruded products having complicated cross-sectional shapes, whereby the shape of the products is limited.
Further, as a method for enhancing the green strength of the EPDM rubber composition, it has been known to formulate an EPDM rubber with higher ethylene content, but the above green strength can be increased at most two-fold by formulation of said EPDM rubber. Also, as a result of this, the properties may be degraded, such as, for example, freeze resistance may be lowered due the crystallization of an ethylene component and the crack growth resistance may be lowered, although the reasons for this are not yet clear.
Further, as shown in (i) to (vi) given below, various rubber compositions and resin compositions have been known in the art, but they are deficient in productivity and processability, and do not have the good mechanical properties of vulcanized products and molded products.
(i) Japanese Unexamined Patent Publication (Kokai) No. 60-139729 discloses a composition containing the following components, a tackifier, a vulcanizable synthetic rubber, a thermoplastic rubber having an amide group, a novolac type phenol resin, and a formaldehyde donor. This composition, as is apparent specifically from Example 13 in the above published specification, comprises a tackifier, EPDM, 6-nylon short fibers, a novolac type phenol resin and hexamethylenetetramine as the essential components, and the above EPDM and the above 6-nylon short fibers are grafted through the novolac type phenol resin.
(ii) Japanese Unexamined Patent Publication (Kokai) No. 61-120855 discloses a composition comprising essential components of a polyphenylene ether, a polyamide and a silane derivative. In this resin composition, although the graft reaction product of the polyphenylene ether and polyamide through the silane derivative may be estimated to exist, neither the polyphenylene ether nor the polyamide are in the form of short fibers, and no chemical bond exists between the rubbery high molecular weight polymer of 50% by weight or less contained in the resin composition and the polyphenylene ether or the polyamide.
(iii) Japanese Unexamined Patent Publication (Kokai) No. 49-104992 discloses an adduct obtained by heating a mixture of an ethylene-.alpha.-olefin-non-conjugated diene copolymer and maleic anhydride.
(iv) Japanese Unexamined Patent Publication (Kokai) No. 51-143061 discloses a poly-phase thermoplastic composition having fine specific polymers dispersed in a matrix of a polyamide.
(v) Japanese Unexamined Patent Publication (Kokai) No. 54-63150 and Japanese Unexamined Patent Publication (Kokai) No. 54-63151 disclose a polyamide type resin composition comprising essential components of a polyamide type resin, an epoxy modified olefin polymer, and a lubricant.
(vi) Japanese Unexamined Patent Publication (Kokai) No. 60-63242 discloses a composition of EPDM and a thermoplastic resin having an aromatic vinyl monomer grafted onto EPDM.
Further as shown in (vii) to (ix) below, various rubber compositions have been known in the art, but these cannot satisfy the required strength and form maintenance characteristic in the unvulcanized state and the physical properties of the vulcanized products such as the modulus, etc.
(vii) Japanese Unexamined Patent Publication (Kokai) No. 55-59950 discloses, for an improvement of the adhesiveness of fibers subjected to RFL treatment and an EPDM rubber composition, a rubber composition in which a methylene group generating agent (methylene donor) is formulated in a mixture of said fibers with said EPDM composition.
(viii) Japanese Unexamined Patent Publication (Kokai) No. 58-56837 discloses, for cancelling weld defects generated during extrusion molding or injection molding of a rubber, a rubber composition in which a methylene acceptor and a methylene donor are formulated in a rubber.
(ix) Japanese Unexamined Patent Publication (Kokai) No. 60-139729 discloses a composition containing essential components of a tackifier, a vulcanizable synthetic rubber, a thermoplastic rubber having an amide group, a novolac type phenol resin, and a formaldehyde donor.
As described above, to prepare a rubber composition with an enhanced form maintenance characteristic of the unvulcanized product and high modulus of the vulcanized product, a large amount of reinforcing agent must be formulated, but formulation of a large amount of reinforcing agent will reduce the flowability and remarkable worsen the processability during rolling and extrusion molding. Moreover, the vulcanized product formulated with a large amount of a reinforcing agent, although exhibiting a high modulus, will generally have a reduced tensile strength.
Recently, as more high-speed roads such as highways are built, a greater high speed durability is demanded of tires. Tires become more susceptible to a standing wave phenomenon as they are run at higher speeds, which may lead to punctures of the tires, as is well known. As one effective approach to an inhibition of the standing wave phenomenon, an improvement of the rigidity at the bead portion has been considered. Also, since a higher modulus of the bead portion also has an effect of reducing the rolling resistance, the bead filler rubber preferably has a high modulus.
On the other hand, once the standing wave phenomenon is generated, the bead portion is greatly deformed and, therefore, from the standpoint of safety, the bead filler rubber must have a good crack growth resistance so that tire punctures are not immediately caused. Thus, as conditions necessary for the bead filler rubber to bring an improvement of the high speed durability of a tire, a high modulus and good crack growth resistance are necessary.
Various methods have been proposed for obtaining a rubber with a high modulus, but these all have their respective problems. The method in which carbon black is formulated in a large amount is not preferable because the rubber cannot be well cohered during the processing steps, the power load is increased during banbury mixer kneading, and difficulties are encountered in tire molding working due to an increased ML of the formulation. The method in which sulfur is formulated in a large amount poses the problems of a blooming of sulfur, a remarkable reduction in physical properties such as flexural crack growth due to an increased crosslinking density, etc. Also, the method of adding a thermosetting resin, which is one method of imparting a high modulus, is subject to severe restrictions in the preparation conditions thereof, such as a prolonged kneading time for obtaining a good dispersion of a thermosetting resin, which is required because of the low compatibility of a large amount of a thermosetting resin with natural rubber or diene type rubber, etc. Also, a kneaded batch formulated with a large amount of a thermosetting resin is relatively hard even when not vulcanized, and therefore, loading becomes greater or the molding workability is reduced during kneading and extrusion. Also, the method of merely formulating short fibers poses the problems of an increased creep or lower fatigue life due to an insufficient bonding between the short fibers and the rubber.
Generally speaking, tires must have an excellent drivability and durability, etc., particularly an excellent wet skid resistance on wet road surfaces from the aspect of safety. Also, based on social demands for a conservation of resources in recent years, to effect a reduction in dynamic loss in tires, tires with a small rotatory resistance, namely with a small energy loss, have been researched and developed. Although the energy loss consumed in freely rotating tires may vary, depending on the structure of the tire, about 1/2 of the total energy is consumed at the tread portion. Accordingly, if the internal consumption in the tread rubber can be reduced, the energy loss during rotation of a tire can be reduced to produce a tire with a low rotational resistance.
Accordingly, attempts have been made to modify the tread rubber to obtain a lower energy loss. However, such a modification of the rubber tends to lower the wet skid resistance. Since an improvement of the rotatory resistance and an improvement of wet skid resistance are antagonistic to each other, various improvements and contrivances have been made to make these matters compatible with each other. One contrivance is to form the tread from two layers of a cap tread and a paste tread. That is, by forming a tire with two layers of a cap tread with a good wet skid resistance and a paste tread with a small energy loss, the wet skid resistance of the tire as a whole is lowered, and the energy loss is also lowered.
As the rubber for the cap tread, in addition to the wet skid resistance and abrasion resistance, a high modulus for a high speed running performance as well as a low viscosity capable of an easy flow even to corners of a complicated tread pattern of a mold are required.
Various methods have been attempted for obtaining a rubber with a high modulus, but all have their problems. The method in which carbon black is formulated in a large amount is not preferable because the rubber cannot be well cohered during the processing steps, the power load is increased during banbury mixer kneading, and difficulties are encountered in tire molding working due to an increased ML of the formulation. The method in which sulfur is formulated in a large amount poses the problems of a blooming of sulfur, and a remarkable reduction in physical properties such as flexural crack growth due to an increased crosslinking density, etc. Also, the method of adding a thermosetting resin, which is one method of imparting a high modulus, is subject to severe restrictions in the preparation conditions thereof, such as a prolonged kneading time for obtaining a good dispersion of a thermosetting resin, which is required because of the low compatibility of a large amount of a thermosetting resin with natural rubber or diene type rubber, etc. Also, a kneaded batch formulated with a large amount of a thermosetting resin is relatively hard even when not vulcanized, and therefore, loading becomes greater or the molding workability is reduced during kneading and extrusion. Also, the method of merely formulating short fibers poses the problems of an increased creep or lower fatigue life due to an insufficient bonding between short fibers and the rubber.
The performances demanded of rubber crawlers running primarily on non-level ground surfaces, such as for vehicles for construction, farm tractors, combat tanks, snow mobiles, etc., are; a resistance to defects or damage of the elastic tread rubber material caused by contact with projections from the ground, and an abrasion resistance.
Although various attempts have been made to improve the cut resistance and abrasion resistance of rubber materials for a rubber crawler, these proved to be unsatisfactory because any improvement of the cut resistance or abrasion resistance (evaluated by picoabrasion) was unsatisfactory, and resulted in an increased Mooney viscosity of the rubber composition or the need for a more complicated molding method, thus reducing the moldability of the rubber materials.
More specifically, even if a rubber composition with an increased amount of carbon black formulated therein is used to improve the cut resistance and abrasion resistance of rubber materials, the cut resistance and abrasion resistance can be improved only a little, and drawbacks arise, such as the Mooney viscosity of the composition becomes higher and the weight of the product is increased. On the contrary, when a rubber composition with a decreased amount of carbon black is used, a drawback arises in that the cut resistance and abrasion resistance are reduced.
On the other hand, the method of adding 5 to 30 PHR of nylon cut fibers to a rubber may improve the durability to some extent, compared with the rubber without the added fiber, but the cut fibers must be oriented vertically to the contact ground surface, probably because cut fibers not bonded with rubber are used, and therefore, the moldability is very bad.